The modern office environment utilises a wide variety of intelligent devices which communicate with each other over a variety of media. The most common arrangement, for both telephony and data network applications, uses cable connections to provide a transmission medium. This requires new cabling, connectors, etc every time a new device is connected to the network. It is accordingly desirable to provide a wireless office system.
A further difficulty is that the available parts of the radio spectrum are limited by government regulation. One approach to providing wireless services is to utilise the available spectrum and power levels within rules for operating emissions from electronic devices. One technique for doing this is to utilise spread spectrum techniques.
Spread-spectrum systems can generally be classified into two types:
*direct sequence spread spectrum PA1 *frequency hopping spread spectrum PA1 * the spectrum is not flat; PA1 * the spectrum is not band-limited; PA1 * band-limiting the spectrum and/or flattening the spectrum requires filtering and can alter the cross-correlation properties of the codes. Special care is needed in the selection of the band pass filters and the demodulation process to preserve these properties. PA1 (a) they are inherently band-limited; PA1 (b) their spectrum is constant over the desired band; PA1 (c) because of (a) their properties are unchanged over band-limited channels. PA1 detecting the channel or channels in which interference is present, PA1 discarding the parts of the de-spreading waveform corresponding to this channel or channels, so as to form a modified de-spreading waveform, and PA1 de-spreading the received signal using the modified de-spreading waveform.
Combination of these types also exist.
The basic operation of a spread spectrum communications system is to take an information signal and spread it in frequency until it occupies a much larger bandwidth than the original information signal. The direct sequence system produces a continuous noise-like signal which contains energy at all frequencies in the spread bandwidth, whereas the frequency hopping system produces a burst signal on discrete frequencies within this band.
In a direct-sequence spread-spectrum system, the information signal is multiplied by the noise-like spreading signal which is a binary waveform. These spreading signals are typically derived from linear feedback shift registers. The binary waveforms consist of ideally random but more usually pseudo-random data at a rate (the chip rate) which is much higher than the information rate. Such a system is shown in prior art FIGS. 1(a) and 1(b).
The simple system in FIG. 1(a) requires a single sequence generator. More signals can be fitted into the same spreading bandwidth if the quadrature channel is utilised as shown in FIG. 1(b). This requires two sequence generators.
As the spectrum of random binary data has a sin(x)/x shape, this spectral shape is typical of spread spectrum systems using a binary data sequence as a spreading waveform. Often a band-limited signal is required, which generally necessitates either filtering of the spreading waveform, or filtering of the spread waveform, or a combination of these.
Some of the undesirable aspects of DS spread spectrum systems for these applications are:
Spread-spectrum receivers often have to contend with interference, much of which is narrow band. FIG. 10 shows the typical scenario of a receiver being presented with a composite signal consisting of the spread spectrum signal and a narrow band interferer. Conceptually it is easy to discriminate between the desired signal and a narrow band interfering signal simply on the basis of bandwidth. An observer using a spectrum analyser could easily discriminate between the wanted and unwanted signals in this case. However, modifying the receiver to only accept the wanted signal and to reduce the effect of the unwanted interferer is a more complex problem.
Various techniques have been proposed for reducing the effect of narrow band interference on spread-spectrum receivers, including placing an adaptive filter before the receiver, and performing the correlation in the frequency domain and removing the effects of the interference there. Neither of these techniques is currently feasible for implementation in low-power portable equipment.
U.S. Pat. No. 5,177,767 and U.S. patent application Ser. No. 967,153 filed on Oct. 27, 1992, relates to the application.